Nowadays, accelerometers have been used in various applications, such as, measuring the magnitude of earthquake and gathering seismic data, detecting the magnitude of collision during a car collision, and detecting the tilting direction and angle of a mobile phone or a game console. As the micro-electro-mechanical systems (MEMS) technology continues to progress, many nano-scale accelerometers have been widely commercially used.
There are two kinds of accelerometers which are commonly used: piezo resistive accelerometers and capacitive accelerometers. Piezoresistive accelerometer, such as Chinese invention patent application with Publication No. CN1748146 and Publication Date of Mar. 15, 2006, usually comprises mass and beams; piezo elements are provided on the beam. The mass moves according to the acceleration, and causes the beam to deform, which also causes the change of resistance. However, under situations when there is no acceleration or the acceleration magnitude is relatively small, the beam will not deform significantly. And there is no significant change in resistance. The accelerometer can only detect acceleration when the magnitude of acceleration is large enough to cause the deformation of the beam. Therefore, such accelerometer has low accuracy.
Capacitive accelerator, such as U.S. Pat. No. 6,805,008, with Publication Date of Oct. 19, 2004, also includes beam and mass. When the acceleration is present, the frame of the accelerometer will move according to the acceleration direction, but due to inertia, there is little displacement for the mass. Thus, the gap distance change between the mass and another electrode causes a change in capacitance. Both kinds of accelerometers are manufactured by micro fabrication technique and have the characteristics of small size and low manufacturing cost. However, the beams are resilient beams, and there are only four beams connecting the mass with the frame. Thus, when the outer frame moves, the displacement of each beam is relatively large. Also, each beam will not create a uniform displacement and deformation, which leads to unsymmetrical vibrational modes.
Furthermore, when packaging the above-mentioned capacitive accelerometer with other microelectronic modules to form an integrated circuit, a package-induced stress is generated due to the difference of coefficients of thermal expansion between different materials. The package-induced stress will directly affect the performance of each component. Compared with other components, MEMS components is more sensible to package-induced stress due to its small size and moveable parts. The package-induced stress may cause a deformation of components in the accelerometer, which leads to inaccurate acceleration detection, and lowers the accelerometer's stability and reliability.